In general, a disk brake mounted on a vehicle includes a brake rotor rotating integrally with a wheel and a brake pad capable of being pressed to a sliding contact surface of the brake rotor. The brake pad moves in a direction to approach the brake rotor when hydraulic pressure in a wheel cylinder corresponding to the wheel is increased, whereas it moves in a direction to move away from the brake rotor when the hydraulic pressure in the wheel cylinder is decreased.
In the vehicle mounted with such a disk brake, when the vehicle travels or turns on a rough road having a large unevenness on the road surface, the wheel receives a reaction force from the road surface and the brake rotor may incline to the brake pad. When the inclined brake rotor contacts the brake pad, a phenomenon called as a knock back, in which the brake pad moves in a direction to move away from the brake rotor, may occur. When a driver operates a brake pedal in a state where the knock back occurs, an operation amount of the pedal is increased as compared with the case where no knock back occurs. In addition, there is a possibility that the driver may feel a sense of discomfort due to the pedal operation. To avoid the occurrence of such knock back, for example, Japanese Patent Application Laid Open No. 07-012145 discloses a brake device.
The brake device disclosed in this patent document includes a hydraulic pressure circuit for connecting a master cylinder, which generates a brake hydraulic pressure corresponding to the operation condition by the brake pedal by the driver, a wheel cylinder, and a holding valve disposed in the hydraulic pressure circuit and driven when the brake hydraulic pressure inside the wheel cylinder needs to be held. In such brake device, after the operation of the brake pedal by the driver is released, the holding valve is driven in order to hold the brake hydraulic pressure inside the wheel cylinder. Thus, even when the brake rotor inclines to the brake pad due to the rough road surface during the travel of the vehicle and the brake rotor contacts the brake pad, the brake hydraulic pressure inside the wheel cylinder is held by the driving of the holding valve. As a result, the movement of the brake pad in the direction to be away from the brake rotor is avoided.
In the brake device disclosed in Japanese Laid-Open Patent Application No. 07-12145, when the brake pedal is not operated, the holding valve continues to be driven in order to hold the brake hydraulic pressure inside the wheel cylinder. Thus, there is a problem that the total power consumption of the brake device becomes extremely large.
During travel of the vehicle, even when the brake rotor inclines to the brake pad, the brake pad does not move in the direction to be away from the brake rotor. Thus, when the vehicle travels in a state where a part of the brake rotor contacts the brake pad, both the brake rotor and the brake pad develop uneven wear. In the case where the uneven wear of the brake rotor and the brake pad grows large, there is a possibility that a vehicle body may vibrate at the time of braking the vehicle, which is called as a brake judder. Further, the driving of the brake device is preferably performed as much as economically as possible in order not to make the brake rotor inclined.